Chronic myelogenous leukemia (CML) is a malignancy of a pluripotent hematopoietic stem cell. The disease is characterized by the presence of a (9;22) chromosomal translocation that results in the production of a 210 kDa fusion protein termed Bcr-Abl. Compared to c-Abl, the Bcr-Abl fusion protein has elevated tyrosine kinase activity. The clinical success of a specific Abl kinase inhibitor, imatinib, has demonstrated the critical dependence of Bcr-Abl function on the kinase activity of the protein. However, imatinib is not capable of eradicating the disease and resistance to single agent imatinib, particularly in patients with advanced phase disease has been common. Thus, greater insight into the molecular mechanism of action of this activated tyrosine kinase may provide additional targets for therapy and an improved therapeutic outcome for patient with CML. Therefore, the goal of this proposal is to learn more about how Bcr-Abl functions to transform cells by determining which signaling proteins and pathways are essential for its function. The first specific aim will analyze Bcr- Abl mutants for defects in transforming abilities in vitro and in vivo. In the second aim, we will follow up on the finding that the main mechanism of acquired resistance to imatinib is mutations in the kinase domain of Abl that impair imatinib binding. These mutants present an ideal opportunity to study the regulation of the Abl kinase. In addition, the mutants allow us an opportunity to address whether these mutations affect the substrate specificity of Abl, the signaling proteins and pathways that are activated, and the transforming properties of these Bcr-Abl mutants. In that last aim, we will follow up on our finding that Bcr-Abl is unable to transform CrkL null cells and that CrkL null cells expressing Bcr-Abl have a variety of signaling defects. These defects will be analyzed to determine the mechanism by which Bcr-Abl is unable to transform CrkL null cells. Combined, these data should offer significant insights into the biology of and the mechanism of transformation by Bcr-Abl.